MCP tool-use execution loop for autonomous agent runs

Autonomous agents with MCP tools now run a bounded tool-use loop: the model may
call tools (executed via the gateway, results fed back) until it returns a final
answer. Gated/DraftOnly agents get the tool catalog as data but never auto-call —
a human-in-the-loop agent never autonomously reaches an external tool.

Extends IModelClient with tool definitions and a tool-use conversation, adds the
OpenAI-compatible tool serialization/parsing plus a deterministic "tooluse" stub
client, and records every tool call in the run trace.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Shared/TeamUp.SharedKernel/Ai/IModelClient.cs

@@ -1,15 +1,42 @@
 namespace TeamUp.SharedKernel.Ai;
 
-/// <summary>One model invocation. The key is passed explicitly (BYOK, server-side only).</summary>
+/// <summary>A tool the model may call (OpenAI "function" tool). Parameters is a JSON-Schema string.</summary>
+public sealed record ModelTool(string Name, string? Description, string ParametersJson);
+
+/// <summary>A tool call the model asked for, to be executed and fed back.</summary>
+public sealed record ModelToolCall(string Id, string Name, string ArgumentsJson);
+
+/// <summary>
+/// One message in a tool-use conversation. Role is user|assistant|tool. An assistant turn may carry
+/// <see cref="ToolCalls"/>; a tool turn carries the result <see cref="Content"/> for <see cref="ToolCallId"/>.
+/// </summary>
+public sealed record ModelMessage(
+    string Role,
+    string? Content,
+    IReadOnlyList<ModelToolCall>? ToolCalls = null,
+    string? ToolCallId = null);
+
+/// <summary>
+/// One model invocation. The key is passed explicitly (BYOK, server-side only). When
+/// <see cref="Messages"/> is set it is the full conversation (for the tool-use loop) and overrides
+/// <see cref="Prompt"/>; <see cref="Tools"/> offers callable tools.
+/// </summary>
 public sealed record ModelRequest(
     string Provider,
     string Model,
     string ApiKey,
     string? Endpoint,
     string Prompt,
-    int MaxTokens = 256);
+    int MaxTokens = 256,
+    IReadOnlyList<ModelTool>? Tools = null,
+    IReadOnlyList<ModelMessage>? Messages = null);
 
-public sealed record ModelCompletion(bool Success, string? Text, string? Error, long LatencyMs);
+public sealed record ModelCompletion(
+    bool Success,
+    string? Text,
+    string? Error,
+    long LatencyMs,
+    IReadOnlyList<ModelToolCall>? ToolCalls = null);
 
 /// <summary>
 /// Provider-agnostic model client. Implemented in Integrations (a router over per-provider HTTP